This invention relates to a metallic nanowire network with a very high surface-to-volume ratio. The ability to form metallic nanowire networks with high surface-to-volume ratios is very desirable for catalytic applications where the catalytic metal is often expensive; significant economic advantage can be obtained by maximizing the fraction of metal atoms accessible to chemical reactants. Platinum and palladium are two examples of highly versatile catalytic metals that are expensive and would benefit from being formed into catalytic materials comprising nanowire networks. For example, nanostructured platinum may be important as an electrocatalyst in proton-exchange-membrane fuel cells, as a catalyst in automotive applications, as a catalyst for industrial reactions, and as a catalyst in solar water-splitting devices. For such applications, it is desirable to have a very high surface-to-volume ratio while also having the catalytic material be readily recoverable when the catalytic process is completed.
Y. Lu and D. Wang, U.S. Pat. No. 7,001,669, report metal-containing nanostructured films by electrodepositing a metal-containing composition within the pores of a mesoporous silica template to form a metal-containing silica nanocomposite. The nanocomposite is annealed to strengthen the deposited metal-containing composition. The silica is then removed from the nanocomposite, e.g., by dissolving the silica in an etching solution to provide a self-supporting metal-containing nanostructured film. The nanostructured films have a nanowire or nanomesh architecture depending on the pore structure of the mesoporous silica template used to prepare the films.
M. Kogiso and T. Shimizu, U.S. Pat. No. 6,858,318, reports a nanowire comprising only metal having an average length of 1 micrometer or more and a method of manufacturing this wire. The method of manufacturing a metal nanowire comprises the step of reducing a nanofiber comprising a metal complex peptide lipid formed from a two-headed peptide lipid comprising valine residues and a metal ion using a reducing agent relative to the two-headed peptide lipid. The method provides a metal nanowire having an averaged diameter of 10-20 nm and average length of 1 micrometer or more; the preferred metal is copper. A nanowire network does not form.
T. Kijima and coworkers report the reduction of metal salts confined to lyotropic mixed surfactant liquid crystals to form Pt, Pd, and Ag nanotubes of 6-7 nm outer diameter. Equimolar amounts of medium and large surfactant molecules are combined into a hexagonal array of cylindrical rod-like micelles approximate 6.9 nm in diameter and the aqueous outer shell of the rod-like micelles is so thick that the reduced metal grows into nanotubes separately within the aqueous shell (T. Kijima, T. Yoshimura, M. Uota, T. Ikeda, D. Fujikawa, S. Mouri, and S. Uoyama, “Noble-Metal Nanotubes (Pt, Pd, Ag) from Lyotropic Mixed-Surfactant Liquid-Crystal Templates,” Angew. Chem. Int. Ed. vol. 43 (2004) pp. 2228-2232).
Attard and coworkers report the formation of microporous platinum particles with a hexagonal nanostructure consisting of cylindrical pores separated by Pt walls using a lyotropic liquid crystalline phase template (G. S. Attard, C. G. Göltner, J. M. corker, S. Henke, and R. H. Templer, “Liquid-Crystal Templates for Nanostructures Metals,” Angew. Chem. Int. Ed. Engl. Vol 36 (1997) pp. 1315-1317). The ternary system consisting of a nonionic surfactant (octaethyleneglycol monohexadecyl), H2PtCl6 or (NH4)2PtCl4, and water forms a lyotropic liquid crystal. Reduction of the Pt(II) employs either a metal less noble that Pt (Fe, Zn, Mg) or hydrazine.
Attard and coworkers also report electrodeposition of metallic mesoporous platinum films from lyotropic liquid crystalline plating mixtures. The plating mixtures were ternary systems consisting of a nonionic surfactant (octaethyleneglycol monohexadecyl), H2PtCl6, and water. Reduction of platinum salts dissolved within the aqueous domains of this hexagonal mesophase lead to platinum whose nanostructure is a cast of the liquid-crystalline phase architecture (G. S. Attard, P. N. Bartlett, N. R. B. Coleman, J. M. Elliott, J. R. Owen, and J. H. Wang, “Mesoporous Platinum Films from Lyotropic Liquid Crystalline Phases,” Science, Vol. 278 (1997) pp. 838-840.
S. Li and coworkers report the formation of an entangled chain network comprised of connected spherical Fe particles in the size range 15-20 nm by reduction of ferrous chloride in a percolating water structure of a gel phase of lecithin/AOT/isooctane/water where the gel phase may be made up of bicontinuous channels of water and the hydrocarbon phase. The particles themselves are spherical and do not take on the morphology of extended structures of the gel (S. Li, V. T. John, G. C. Irvin, B. Simmons, G. D. McPherson, and W. Zho, “The use of organic templates to develop biomimetic chain structures of magnetic nanoparticles,” J. Appl. Phys vol. 87 (2000) pp 6211-6213).
A mixture of nanospheres and nanorods of Cu have been formed by reduction of Cu(AOT)2 by hydrazine in the interconnected cylinders phase of the Cu(AOT)2/isooctane/water system, which is a surfactant/organic solvent/water ternary system (M. P. Pileni, “Mesostructured Fluids in Oil-rich Regions: Structural and Templating approaches,” Langmuir Vol. 17 (2001) pp. 7476-7486). The interconnected cylinders phase corresponds to the bicontinuous phase. A nanowire network does not form.